The present invention relates to a coherent radiation source generating a beam having a regulatable propagation direction.
In a large number of laser applications, it is necessary to be able to rapidly change the beam direction, e.g. in optical memories, photo-composition and wide screen television. The most conventional and still very frequently used deflectors are rotary or vibratory mirrors. However, other devices without moving parts have been proposed and these are mainly constituted by acoustic and electrooptical deflectors.
If .alpha..sub.m is the maximum deflection angle and .DELTA..alpha. the angle between two separable directions, it is possible to geometrically characterize a deflector by the number N of separate directions which can be imparted to the beam, giving N=.alpha..sub.m /.DELTA..alpha.. In all cases, .DELTA..alpha. is limited by the diffraction corresponding to the beam aperture.
With regards to the operating mode, a distinction is made between sequential and random deflections. The first mode corresponding to a continuous angular scan can be characterized by the number of scanned positions, per second. For example, a television picture with 625 lines and 25 images per second requires the successive scanning of 1.3.times.10.sup.7 points per second. The random deflection necessary, for example, for giving access to an optical memory is characterized by the mean access time to a given direction or position.
On considering an electrooptical photosensitive medium, the interference between two incident beams in this medium ensures the generation of a system of lines having an appropriate spacing and orientation for deflecting the laser beam. This interference takes place in a three-dimensional medium. The physical characteristics of this medium and in particular the refractive index are spatially modulated by a system of fringes due to the interference of said two incident beams. As a result of this spatial modulation, inducing by gradient a system of lines, any incident beam can be diffracted in this way in a chosen direction. The two writing beams emanate from the same source, which can be e.g. a helium-cadmium laser after traversing a beam splitter.